1. Field of the Invention
The present invention relates to a display device for effecting a display by applying a drive signal to a pixel electrode through a switching element, and more particularly, relates to an active matrix display device in which a plurality of pixel electrodes are arranged in a matrix so as to effect a high density display.
2. Description of the Prior Art
Conventionally, in a liquid crystal display device, an EL (electro luminescence) display device, a plasma display device, and the like, a display pattern is formed on a screen thereof by selectively driving pixel electrodes arranged in a matrix.
Such selective driving of the pixel electrodes can be effected by a known active matrix drive system, in which individual pixel electrodes are connected to their respective switching elements so as to be driven independently. For the switching elements, TFTs (thin film transistors), MIM (metal-insulator-metal) elements, MOS (metal-oxide-semiconductor) transistors, diodes, varistors, or the like can be used. Each switching element switches the application of a voltage between the corresponding pixel electrode and a counter electrode facing the pixel electrode, so as to optically modulate a display medium, such as a liquid crystal, an EL layer, and a plasma emitter, interposed between the electrodes. Such optical modulation is visually recognized as a display pattern. This active matrix drive system can effect a display of high contrast, and therefore, has been applied to, for example, a liquid crystal television set, a word processor, and a computer terminal display device.
In the above conventional active matrix display devices, when a switching element is defective, a pixel electrode connected to the switching element can not receive an otherwise applied signal. As a result, the pixel with such a pixel electrode is recognized as a point defect on the display screen. Such a point defect deteriorates the quality of the display, and the yield of manufacture is lowered.
Generally, the inferiority of a pixel including a point defect and a line defect is largely caused by the following two reasons: (1) a pixel electrode can not be sufficiently charged when a switching element is on (hereinafter referred to as an "ON defect", and (2) the charge in a pixel electrode is leaked when a switching element is off (hereinafter referred to as an "OFF defect").
The ON defect is caused by a defective switching element. On the other hand, the OFF defect is caused by two types of electrical leakages; a leakage between a pixel electrode and a source bus through a switching element, and a leakage between a pixel electrode and a source bus or a gate bus. In either case of the ON defect or the OFF defect, the voltage applied between the pixel electrode and the counter electrode does not reach a required value. As a result, the point defect is observed as a luminescent spot on the display screen when a normally white mode (a display mode in which the light transmittance is maximum when the voltage applied to the liquid crystal is zero) is used, and as a black spot on the display screen when a normally black mode (a display mode in which the light transmittance is minimum when the voltage applied to the liquid crystal is zero) is used.
If a point defect is detected at the stage of manufacturing the substrate having the switching elements formed thereon, in some cases, the defective pixel can be corrected by a technique such as laser cutting. However, it is quite difficult to detect a point defect among a huge number of pixels during the manufacture of the substrate; practically impossible in the case of mass production, considering time and cost required. Especially, it is completely impossible to detect a point defect when a large-scale display panel including hundreds of thousands of pixels is manufactured.
To cope with the above problems display devices as shown in FIGS. 4 and 6 are proposed (Japanese Laid-Open Patent Publication Nos. 4-16929, 4-16930, 4-19618, and 3-24524), in which an electrical signal for detection is applied to buses at the stage where a substrate having switching elements formed thereon and a counter substrate have been attached together with a liquid crystal sealed therebetween, so that a point defect can be easily detected by visual observation. According to such display devices, the detected defective pixel can be corrected from outside by a technique such as laser cutting.
A conventional display device in FIG. 4 comprises gate buses 21 disposed in one direction on one of paired substrates facing each other and source buses 23 disposed in a direction transverse to the above direction. A pixel electrode 41 is disposed in each rectangular area defined by the adjacent gate buses 21 and the adjacent source buses 23.
A gate bus branch 22 is formed as an extension from the gate bus 21, and comprises a portion functioning as a gate electrode for a TFT 31 and a portion narrower than the above portion. The TFT 31 which functions as a switching element comprises a drain electrode 33 electrically connected to the pixel electrode 41, a source electrode 32 electrically connected to the source bus 23 which are formed over the gate bus branch 22, and the gate electrode of the gate bus branch 22.
In the above-described display device, the detection of a point defect and correction of the defective pixel is performed in the following procedure. The substrate having the TFT 31 formed thereon and the counter substrate are attached together with a liquid crystal sealed therebetween. In this condition, appropriate signals are applied to the gate buses 21, the source buses 23 and a counter electrode formed on the counter substrate, so as to detect any possible point defect by visual observation. When a point defect is detected, the defective pixel is corrected by a technique such as laser cutting.
Referring to FIG. 5, the correction of the defective pixel is performed as follows. First, the narrow portion of the gate bus branch 22 is cut by laser radiation to separate the TFT 31 from the gate bus 21. An area 51 enclosed by a double line in the figure shows the portion radiated by the laser. In this way, the gate electrode of the TFT 31 is electrically disconnected from the gate bus 21.
Subsequently, part of an overlapping portion of the source electrode 32 and the gate electrode of the TFT 31 and part of an overlapping portion of the drain electrode 33 and the gate electrode of the TFT 31, shown as areas 52 and 53 enclosed by double lines in FIG. 5, respectively, are shot through by laser radiation through the transparent substrate. The overlapping electrodes are then electrically connected to each other through the edges of the areas 52 and 53, respectively. More specifically, the source electrode 32 and the gate electrode of the TFT 31, and the drain electrode 33 and the gate electrode of the TFT 31 are electrically connected. This means that the source bus 23 and the pixel electrode 41 are electrically connected through the gate electrode of the TFT 31. Thus, since the pixel electrode 41 is short-circuited with the source bus 23, it keeps the same potential as the source signal. As a result, the point defect becomes unobtrusive, and thus the defective pixel can be corrected.
Another conventional display device shown in FIG. 6 comprises the TFT 31 formed over the gate bus branch 22 extended from the gate bus 21 including a portion of the gate bus branch 22 functioning as a gate electrode, as in the display device shown in FIG. 4. In this case, however, a redundant structure for short-circuiting the pixel electrode 41 with the source bus 23 is additionally formed. The redundant structure comprises an extrusion 46 extended from the source bus 23, a conductive portion 47 overlapping the portion of the extrusion 46, and a conductive piece 48 overlapping the conductive portion 47 and electrically connected to the pixel electrode 41. Insulating films are interposed between the extrusion 46 and the conductive portion 47 and between the conductive portion 47 and the conductive piece 48, so as to be insulated from each other.
Referring to FIG. 7, the detection of a point defect and correction of the defective pixel is performed as follows. At the stage that the substrate having the TFT 31 formed thereon and the counter substrate have been attached together with the liquid crystal sealed therebetween, appropriate signals are applied to gate buses 21, source buses 23 and a counter electrode formed on the counter substrate, so as to detect any possible point defect by visual observation.
When a point defect is detected, part of an overlapping portion of the extrusion 46 of the source bus 23 and the conductive portion 47 and part of an overlapping portion of the conductive portion 47 and the conductive piece 48, shown as areas 54 and 55 enclosed by double lines in FIG. 7, respectively, are shot through by laser radiation through the transparent substrate. In this way, the extrusion 46 and the conductive portion 47, and the conductive portion 47 and the conductive piece 48 are electrically connected. Since the conductive piece 48 has already been electrically connected to the pixel electrode 41, the pixel electrode 41 can be short-circuited with the source bus 23 by the above-described laser radiation at two places, whereby the pixel electrode 41 can keep the same potential as the source signal. As a result, the point defect becomes unobtrusive, and thus the defective pixel can be corrected.
As is apparent from the above description, the conventional display devices are constructed so that the pixel electrode 41 can be short-circuited with the source bus 23 through which signals are applied to the pixel electrode 41, thereby to make a point defect unobtrusive and thus to correct the defective pixel.
In the above-described display devices, however, the correction of a leakage between the source bus 23 and the pixel electrode 41 may not be possible depending on the place where the leakage occurs. For example, as shown in FIG. 8, when a leakage occurs between the pixel electrode 41 and the source bus 23 through which signals are applied to the pixel electrode 41 at a portion 81 shown by slant lines, a current flows from the pixel electrode 41 to the source bus 23 when the TFT 31 is off. As a result, a pixel with the pixel electrode 41 is recognized as a point defect on the display. In this case, such a defective pixel can be corrected by short-circuiting the pixel electrode 41 with the source bus 23 by the laser radiation onto the TFT 31 as described above.
However, a problem arises when a leakage occurs between the pixel electrode 41 and the source bus 23 through which signals are not applied to the pixel electrode 41 at a portion 82 shown by slant lines. When the laser processing as described above is performed on the portions of the TFT 31, a leakage occurs between the pixel electrode 41 and the source bus 23 through which signals are applied, in addition to the leakage between the pixel electrode 41 and the source bus 23 through which signals are not applied to the pixel electrode 41. This means that a leakage between the adjacent source buses 23 occurs through the pixel electrode 41. As a result, signals passing in the adjacent source buses 23 are mixed with each other through the pixel electrode 41. This produces a line defect, which is more serious than the point defect.
The same problem, (i,e, a line defect) occurs for the device shown in FIG. 6 when a leakage occurs between the pixel electrode 41 and the source bus 23 as described above.